Sorption measurements of moisture-equilibrated shale with the consideration of water vapor pressure

High-pressure methane sorption isotherms measured at 303.15,323.15 and 353.15 K and pressure up to 20 MPa were measured on dry and moisture-equilibrated shale based on an improved volumetric sorption equipment. The sorption data were fitted by a modified Langmuir equation. Assuming that the experimental gas is the mixture of methane and water vopor in the moisture-equilibrated shale sorption experiment, the sorption capacity of methane accounting for water vapor pressure were recalculated, and the importance of water vapor pressure on shale sorption was discussed basing on thermodynamics of sorption. The results show that the Langmuir maximum sorption capacities of moisture-equilibrated shale with/without consideration of water vapor pressure are 0.053 and 0.039 mmol/g, respectively. Methane sorption capacity without consideration of water vapor pressure is underestimated by 26.4%. The isosteric heat of adsorption for dry shale and moisture-equilibrated shale with/without consideration of water vapor pressure are 11.6, 10.7 and 12.3 kJ/mol, respectively. Since water preferentially occupies the higher energy sites, the presence of moisture is expected to reduce rather than increase the isosteric heat of adsorption. Therefore, the thermodynamic parameter corrected by water is more consistent with thermodynamic consistency. Under the condition of formation, the effect of the water vapor pressure on the sorption of shale can be divided into two stages: (1) shallow burial stage (depth of stratum less than 700 m), this stage is dominated by the pressure gradient. The greater the pressure gradient, the greater the effect of the water vapor pressure on the sorption of shale; (2) deep burial stage (depth of stratum more than 700 m), this stage is controlled simultaneously by pressure gradient and temperature gradient. The greater the pressure gradient and the smaller the temperature gradient, the greater the effect of the water vapor pressure on the sorption of shale.